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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is
 not a constant expression
Date: Thu, 15 Oct 2020 16:45:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95942

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|NEW                         |RESOLVED

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
C11 says,

....and
  offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the v=
alue
of which is the offset in bytes, to the structure member (designated by
member-designator), from the beginning of its structure (designated by type=
).
The type and member designator shall be such that given
  static type t;
then the expression &(t.member-designator) evaluates to an address constant.
(If the specified member is a bit-field, the behavior is undefined.)

And __builtin_offsetof is only documented to be usable as an implementation=
 of
the offsetof macro.

Since e is not constant, the expression above is not an address constant, so
the offsetof expression is ill-formed.  The change mentioned in comment #2
fixed this and various related issues with wrongly treating parameters as
sufficiently constant.=